Process for treating spent iron-containing chromium plating solution to remove iron values contained therein and regenerate the solution

ABSTRACT

Novel and improved process for the treatment of exhausted chromium-plating solutions or electrolytes wherein all chromium values contained in the exhausted chromium-plating electrolyte can be converted to a water-soluble chromium chromate complex compound through treatment with a chromium chromate complex compound-forming reagent and, at the same time, all iron values which are present in the exhausted electrolyte and have a detrimental influence on the chromium-plating operation are converted to ferric hydroxide which precipitates and is easily removable. The thus obtained aqueous chromium chromate complex compound solution can be, in one way, recycled to the chromiumplating system, as a souce of trivalent chromium which is necessary, in a small amount, for preparing a useful chromiumplating electrolyte or solution; or can be, in another way, employed as an anode electrolyte of a diaphragm electrolysis to produce a fresh chromium-plating electrolyte.

United States Patent Fuj PROCESS FOR TREATING SPENT IRON-CONTAINING CHROMIUM PLATING SOLUTION TO REMOVE IRON VALUES CONTAINED THEREIN AND REGENERATE TI-IE SOLUTION Inventor: Akira Fujii, Kawasaki, Japan Assignee: Kabushiki Kaisha Fuji Kuromu Sha,

Japan Filed: Feb. 12, 1974 Appl. No.: 441,890

Related U.S. Application Data [63] Continuation of Ser. No. 240.763. April 3. 1972.

abandoned.

[52] U.S. Cl. 204/51; 204/DIG. 13; 210/50; 210/59 [51] Int. Cl. C25D 3/04; B0lD 9/02 [58] Field of Search 204/51, 105 R; 210/50, 210/59 [56] References Cited UNITED STATES PATENTS 2.803.594 8/1957 Westby 204/105 R 2.841.540 7/1958 Smith 204/51 3.097.064 7/1963 Caldwell 204/51 X 3.444.060 5/1969 Rothmunn et a1. 204/105 R OTHER PUBLICATIONS L. F. Hamilton et al.. Calculations of Analytical Chemistry. p. 373. (1947). a

Primary Examiner-Ci. L. Kaplan Attorney, Agent, or Firm-Armstrong, Nikaido 8t Wegner [57] ABSTRACT Novel and improved process for the treatment of exhausted chromium-plating solutions or electrolytes wherein all chromium values contained in the exhausted chromium-plating electrolyte can be converted to a water-soluble chromium chromate complex compound through treatment with a chromium chromate complex compound-forming reagent and, at the same time. all iron values which are present in the exhausted electrolyte and have a detrimental influence on the chromium-plating operation are converted to ferric hydroxide which precipitates and is easily removable. The thus obtained aqueous chromium chromate complex compound solution can be, in one way. recycled to the chromium-plating system. as a souce of trivalent chromium which is necessary. in a small amount, for preparing a useful chromium-plating electrolyte or solution; or can be. in another way. em-

ployed as an anode electrolyte of a diaphragm electrolysis toproduce a fresh chromium-plating electrolyte.

2 Claims, No Drawings PROCESS FOR TREATING SPENT IRON-CONTAINING CHROMIUM PLATING SOLUTION TO REMOVE IRON VALUES CONTAINED THEREIN AND REGENERATE THE SOLUTION This application is a continuationof application Ser. No. 240,763 filed Apr. 3, 1972 and now abandoned.

This invention relates to a process for the treatment of exhausted chromium-plating solution on electrolytes and more particularly to a novel and improved process for the treatment of exhausted chromium-plating solutions or electrolytes wherein all chromium values contained in the exhausted chromium-plating electrolytes are converted to a water-insoluble chromium chromate complex compound and, at the same time, an iron values contained in the cromium-plating solutions are converted to ferric hydroxide which is water-insoluble.

conventionally, the chromium-plating wastes are treated with a reducing agent and then, an alkali is added thereto to form chromium hydroxide as a precipitate, which is discharged as a sludge. The supernatant liquid is also discharged. On the other hand, the exhausted chromium-plating electrolyte is conventionally regenerated'by subjecting it to a diaphragm electrolysis or treating it with an ion exchange resin. Yet, the former method not only requires a long period of time for pletely regenerated electrolyte which can be used as a plating electrolyte to concentrate the diluted electrolyte, leading to economical difficulty in practicability.

. Additionally to say, there has not been proposed any practical method for making the exhausted chromiumplating solutions or electrolytes harmless and dischargeable.

-In order to eliminate such difficulties in treatment of the exhausted chromium-plating electrolytes as well as the plating wastes, the present inventor has made an extensive and intensive study. As a result, it has been found that all chromium values contained in the exhausted chromium-plating electrolyte can be converted to a water-soluble chromium chromate complex com pound through treatment with a chromium chromate complex compound-forming reagent and, at the same time, all iron values which are present in the exhausted electrolyte and have a detrimental influence on the chromium-plating operation are converted to ferric hydroxide which precipitates and is easily removable. It has also been found that the thus obtained aqueous chromium chromate complex compound solution can be, in one way, recycled to the chromium-plating sys tem, as a source of trivalent chromium which is necessary, in a small amount, for preparing a useful chromium-plating electrolyte or solution; or can be, in another way, employed as an anode electrolyte of a diaphragm electrolysis to produce a fresh chromium-plating electrolyte.

The present invention has been made on the basis of such novel findings.

Accordingly, it is one object of the present invention to provide a process for the treatment of exhausted chromium-plating solution or electrolyte whereby the exhausted chromium-plating solution can be effectively regenerated.

It is another object of" the present invention to provide a process of the kind described above, wherein chromium-plating wastes can be effectively utilized.

It is a further object of the present invention to provide a process of the character described which is simple in operation and can be carried out at a low cost.

' mium chromate complex compound-forming reagent to convert all chromium values contained in. said exhausted chromium-plating solution to a chromium chromate complex compound and, at the same time, convert all iron values contained in said exhausted chromium-plating solution to ferric hydroxide; and subjecting the resulting mixture to filtration thereby to separate an aqueous chromium chromate complex compound solution from the ferric hydroxide.

The exhausted chromium-plating solution or electrolyte usually contains iron values which-have a detrimental influence on the chromium-plating operation. When the exhausted chromium-plating solution or electrolyte is treated with a chromium chromate complex compound-forming reagent (which will be men tioned later), all chromium values contained in the exhausted electrolyte are converted to a water-soluble chromium chromate complex compound containing trivalent chromium and hexavalent chromium and, at the same time, all iron values contained in the exhausted electrolyte are converted to ferric hydroxide which is water-insoluble. The chromium chromate complex compound is obtained as an aqueous solution through filtration. Whilst, the ferric hydroxide is obtained as a filtration residue and can bereadily converted to ferric oxide by firing. The resulting, ferric oxide is harmless and hence, it can be discharged without causing any environmental pollution. On the other hand, the aqueous chromium chromate complex compound solution can be, as such, recycled to the chromium-plating electrolyte. The aqueous chromium chromate complex compound can also be employed as an anode electrolyte of a diaphragm electrolysis to produce a fresh chromium-plating electrolyte. In such diaphragm electrolysis, an aqueous solution of sodium dichromate, potassium dichromate, ammonium dichromate, sodium chromate, potassium chromate, ammonium chromate and the like, which may be employed alone or in mixture, may be used as a cathode electrolyte. In this connection, it is noted that, if the aqueous chromium chromate complex compound solution contains a small amount of iron values, the iron values are transferred to the cathode compartment through the diaphragm during the electrolysis, leading to complete removal of the iron values from the aqueous chromium chromate complex compound solution.

Examples of the chromium chromate complex compound-forming reagents include chromium trihydroxide and an alcohol represented by the formula ROI-l wherein R is an alkyl group having 1 to 3 carbon atoms. The chromium trihydroxide may be employed in an equivalent amount to that of the chromic anhydride in the exhausted solution. The alcohol may be employed in an amount of 400 ml. per 1 kg. of the chromic anhydride in the exhausted solution. In this connection, it is to be noted that the chromium trihydroxide, which is obtained from the aforementioned sludge formed by treating the chromium-plating wastes according to the conventional method, may be employed as a chromium chromate complex compound-forming reagent in, the present process. Accordingly, the harmful sludge can be effectively utilized with such great advantage that the possible environmental pollution could be eliminated.

The essential features of this invention can be illustrated by the following flow sheet.

Exhausted chromiumplating electrolyte 41 Coh l I Chromium chromate complex compound and Fe(OH) Fe(OH).-, firing discharge lron oxide as residue) Filtration chromium chromate complex compound (as filtrate) of diaphragm electrolysis To Anode compartment regenerated chromiumplating electrolyte EXAMPLE 1 To 200 liters 'of exhausted chromium-plating electrolyte including 200 g/l of chromic anhydride, 14.3 g/l of iron, 1.8 g/l of sulfuric acid and 3.5 g/l of trivalent chromium value were gradually added 16 liters of ethyl alcohol. Resultantly, said exhausted chromium-plating electrolyte was spontaneously heated to boiling due to an exothermic reaction caused by reduction so that the reaction was promoted and completed in l or 2 hours. Then, the electrolyte was let to be cooled till it was di vided into two layers. Thus, an iron hydroxide precipitated in the lower layer. Subsequently, the liquid mate rial thus divided into two layers was charged and centrifugalized in a central chamber of a centrifugal separator partitioned by a diaphragm, thereby to completely separate iron hydroxide residue from an aqueous chromium chromate complex compound solution. The thus obtained iron hydroxide residue was burnt at a lower temperature to become an iron oxide which is a harmless waste unsoluble to water. The analysis of the chromium chromate complex compound solution shows it was composed of 48 g of trivalent chromium value, 100 g of chromic anhydride and 0.05 g of iron per liter of the solution. A part of the solution was added to a chromium-plating electrolyte lacking for trivalent chromium value to be reemployed for a further chromium-plating and the purpose was achieved readily without any problem.

EXAMPLE 2 25 Liters of chromic chromium solution as obtained in Example 1 was added to liters of chromiumplating electrolyte having a composition of 200 g/l of chromic anhydride, 2 g/l of sulfuric acid and 1 liter of water. The thus obtained 100 liters of plating electrolyte was subjected to electrolytic oxidation in an ordinary manner (10V, 100A and 30h), whereby the trivalent chromium value was reduced from 12 g/] to 3.5 g/l in its amount. The thus obtained electrolyte had no difficulty to effect chromium-plating.

EXAMPLE 3 To 1 liter of plating electrolyte including 200 g/l of chromic anhydride, 10 g/l of iron, 2 g/l of sulfuric acid, 1 liter of water and 3.5 g/l of trivalent chromium value were added 200 g of chromium hydroxide. Then the electrolyte was heated and reacted at C to form solution including 400 g of chromium chromate complex compound. The ferrous compound was pre-- cipitated as iron hydroxide.

As mentioned above, according to the present invention, exhausted chromium-plating electrolyte can be treated extremely easily, a harmful ferrous compound can be completely eliminated and available chromic compound in the exhausted chromium-plating electrolyte can be completely recovered for further chromium-plating. The present invention further has advantages that it can solve the long problem of environmental pollution by treating the waste at an extremely low cost and economical advantage owing to its complete recovery of chromic anhydride.

What is claimed is:

1. A process for the treatment of iron containing chromium-plating solutions to remove said iron therefrom which comprises treating the chromium-plating solution with chromium trihydroxide to convert all chromium values contained in said chromium-plating solution to a chromium chromate complex compound and, at the same time, convert all iron values contained in said chromium-plating solution to ferric hydroxide; and subjecting the resulting mixture to filtration thereby to separate an aqueous chromium chromate complex compound solution from the ferric hydroxide.

2. A process for the treatment of iron containing chromium plating solutions to remove said iron therefrom and regenerate a chromium plating electrolyte which comprises (i) treating said chromium-plating solution with chromium trihydroxide whereby the chromium values contained in the solution are converted to a chromium chromate complex while simultaneously converting the iron values in the solution to ferric hydroxide; (ii) subjecting the resultant mixture to filtration whereby chromium trihydroxide is separated from the ferric hydroxide; (iii) charging said aqueous chromium chromate complex compound solution into an anode compartment of a diaphragm cell followed by electrolysis, thereby to form a regenerated chromiumplating electrolyte. 

1. A PROCESS FOR THE TREATMENT OF IRON CONTAINING CHROMIUMPLATING SOLUTIONS TO REMOVVE SAID IRON THEREFROM WHICH COMPRISES TREATING THE CHROMIUM-PLATING SOLUTION WITH CHROMIUM TRIHYDROXIDE TO CONVERT ALL CHROMIUM VALUES CONTAINED IN SAID CHROMIUM-PLATING SOLUTION TO A CHROMIUM CHROMATE COMPLEX COMPOUND AND, AT THE SAME TIME, CONVERT ALL IRON VALUES CONTAINED IN SAID CHROMIUM-PLATING SOLUTION TO FERRIC HYDROXIDE, AND SUBJECTING THE RESULTING MIXTURE TO FILTRATION THEREBY TO SEPARATE AN AQUEOUS CHROMIUM CHROMATE COMPLEX COMPOUND SOLUTION FROM THE FERRIC HYDROXIDE.
 2. A PROCESS FOR THE TREATMENT OF IRON CONTAINING CHROMIUM PLATING SOLUTIONS TO REMOVE SAID IRON THEREFROM AND REGENERATE A CHROMIUM PLATING ELECTROLYTE WHICH COMPRISES (I) TREATING SAID CHROMIUM-PLATING SOLUTION WITH CHROMIUM TRIHYDROXIDE WHEREBY THE CHROMIUM VALUES CONTAINED IN THE SOLUTION ARE CONVERTED TO A CHROMIUM CHROMATE COMPLEX WHILE SIMULTANEOUSLY CONVERTING THE IRON VALUES IN THE SOLUTION TO FERRIC HYDROXIDE, (II) SUBJECTING THE RESULTANT MIXTURE TO FILTRATION WHEREBY CHROMIUM TRIHYDROXIDE IS SEPARATED FROM THE FERRIC HYDROXIDE, (III) CHARGING SAID AQUEOUS CHROMIUM CHROMATE COMPLEX COMPOUND SOLUTION INTO AN ANODE COMPARTMENT OF A DIAPRAGM CELL FOLLOWED BY ELECTROLYSIS, THEREBY TO FORM A REGENERATED CHROMIUM-PLATING ELECTROLYTE. 